With the recent development of electronic technologies, cellular phones, laptop personal computers, digital cameras, audio-visual equipment, personal digital assistants and the like have become compact, and have spread widely as mobile electronic devices. Such mobile electronic devices are conventionally driven by a secondary battery. The successive appearance of new secondary batteries, that is, a seal lead battery, Ni/Cd battery, Ni/hydrogen battery and Li ion battery in the order of mention and technologies for size/weight reduction and attaining high energy density have supported the development of them. In any of these secondary batteries, a battery activating substance and a battery structure permitting a high capacity have been developed in order to heighten the energy density. Efforts have been made to prolong the serviceable time of the secondary battery per charging. These secondary batteries however have to be charged after consumption of a predetermined volume of power, and they therefore require charging equipment and a relatively long charging time. Thus, many problems have still remained in continuous operation of mobile electronic devices for long hours. Mobile electronic devices tend to require a power source having a higher output power density and a higher energy density to meet an increasing information amount and increasing processing speed in future. There is therefore an increasing demand for a small-sized power generator (micro power generator) which does not need charging.
A fuel cell power source system is considered as a system capable of meeting such a demand. A fuel cell directly converts chemical energy of fuel to electrical energy through electrochemical reaction. Accordingly, there is a high possibility of this system being realized as a small-sized power generating device, because it does not need a power output portion such as power generator using an internal combustion engine such as ordinarily employed engine power generator. In addition, a fuel cell enables continuous power generation only by replacing fuel or refilling fuel. It is not necessary to interrupt the operation of a mobile electronic device when its battery such as a secondary battery is charged. Under such a demand, a polymer electrolyte fuel cell (PEFC) using a perfluorocarbon sulfonic acid resin electrolyte membrane and carrying out oxidation of a hydrogen gas at an anode and reduction of oxygen at a cathode is under development. When it is used as a power source of a mobile electronic device, the volume of a fuel tank must be increased because of a low volumetric energy density of a hydrogen gas as fuel. This cell is therefore unsuited for size reduction.
Compared with a gas, liquid fuel is advantageous as fuel for fuel cell of small-sized devices because its density is higher than that of a gas. A fuel cell using liquid fuel such as methanol, ethanol, propanol, dimethyl ether or ethylene glycol is considered promising as a power source for small-sized devices which can be operated for long hours.
By using a direct methanol fuel cell (which will hereinafter be abbreviated as “DMFC”) which is a standard one as an example of a fuel cell power source using liquid fuel, the principle of a DMFC power source will next be described. The DMFC is composed of a cell, a fuel container, a fuel supply apparatus and air or an air supply apparatus. The cell is obtained by connecting, in series or if necessary in parallel to each other, unit cells having a porous anode electrode and a cathode electrode on respective sides of a solid electrolyte, an anode diffusion layer and a cathode diffusion layer disposed outside of these electrodes, and an anode endplate and a cathode endplate disposed on the outside of the anode diffusion layer and cathode diffusion layer, respectively and having a current collecting function. When methanol is used as fuel, methanol passing through the anode diffusion layer and coming in contact with the catalyst at the anode electrode reacts with water in accordance with the reaction formula (1) and dissociates into carbon dioxide, hydrogen ions and electrons.CH3OH+H2O→CO2+6H++6e−  (1)
The hydrogen ions reach the cathode electrode, passing through an electrolyte in the anode electrode and a solid electrolyte membrane between the anode electrode and cathode electrode. Oxygen fed to the cathode electrode and electrons entering from the outside circuit are brought into contact with a cathode catalyst, whereby reaction occurs in accordance with the following reaction formula (2) to generate water.6H++3/2O2+6e−3H2O  (2)
Electrons emitted from methanol pass through a catalyst carrier in the anode electrode and the anode diffusion layer, collected in the anode current collector and introduced into the outside circuit. From the outside circuit, they flow into the cathode electrode, passing through the cathode current collector and cathode diffusion layer. As a result, electrons move from the anode electrode toward the cathode electrode in the outside circuit, leading to power generation.
In the total chemical reaction occurring upon power generation, methanol is oxidized by oxygen and produces a carbon dioxide gas and water as shown below in the reaction formula (3). Its reaction formula is therefore formally similar to that of flaming combustion of methanol. In a fuel cell using an aqueous methanol solution as fuel, power generation occurs as direct conversion of chemical energy of methanol to electrical energy in the above-described electrochemical reaction.CH3OH+3/2O2→CO2+3H2O  (3)
In the conventional DMFC, carbon dioxide generated in the reaction formula (1) and carbon monoxide which is an intermediate product accumulate in the anode electrode, anode diffusion layer or anode endplate and disturb the supply of fuel, which deteriorates a power generation efficiency, decreases an effective surface area of a catalyst to reduce an output voltage, and increases an internal pressure to disturb supply of fuel, resulting in a reduction in the output voltage. Smooth discharge of a gas adsorbed as a foam to the electrode surface, diffusion layer or anode endplate to prevent an increase in the internal pressure is therefore indispensable for stable supply of fuel and attaining a predetermined output voltage. In particular, in a passive type DMFC not using a forced mechanism such as pump for fuel supply, smooth discharge of a gas such as carbon dioxide generated at an anode and prevention of an increase in internal pressure are necessary and indispensable.
There have been proposed methods of releasing a gas such as carbon dioxide remaining at an anode electrode, for example, by adding a defoaming agent to an aqueous methanol solution, incorporating a defoaming agent in an electrode, or, as described in Japanese Patent Unexamined Publication No. 2004-039307, incorporating a trapping agent of carbonate ions in an anode electrode or fuel supply portion. In the method of adding a defoaming agent to an aqueous methanol solution, however, the amount of the defoaming agent increases owing to accumulation of it with the passage of time, which poisons an anode catalyst, thereby deteriorating the catalyst capacity of the anode electrode. In the method of incorporating a defoaming agent in an electrode, the defoaming agent poisons an anode catalyst and lowers the catalyst capacity, which leads to drawbacks such as increase in the internal pressure and reduction in the fuel supply capacity. Another method proposed is to release carbon dioxide remaining at an anode electrode by oscillation or forced circulation of fuel by a pump. Such a method is not satisfactory when the recent tendency to size/weight reduction is considered, because it consumes power for oscillation or fuel circulation.
There is accordingly an eager demand for the establishment of a method for releasing a carbon dioxide gas which method is effective for long hours, in order to prevent an increase in internal pressure or reduction in the fuel supply capacity which will otherwise occur by a gas adsorbed as a foam to the anode diffusion layer or anode endplate.
An object of the present invention is, in order to prevent an output reduction of a fuel cell which will otherwise occur owing to the disturbance of fuel supply by the accumulation, in an anode diffusion layer or anode endplate, of a gas such as carbon dioxide produced by the reaction between methanol and water at an anode electrode or carbon monoxide which is an intermediate product of the carbon dioxide, to smoothly discharge the gas adsorbed in the form of foam to the anode diffusion layer or anode endplate.